Humoral immunity and cell-mediated immunity are the two major branches of the mammalian immune response. Humoral immunity involves the generation of antibodies to foreign antigens. Antibodies are produced by B-lymphocytes. Cell-mediated immunity involves the activation of T-lymphocytes which either act upon infected cells bearing foreign antigens or stimulate other cells to act upon infected cells. Both branches of the mammalian immune system are important in fighting disease. Humoral immunity is the major line of defense against bacterial pathogens. In the case of viral disease, the induction of cytotoxic T lymphocytes (CTLs) appears to be crucial for protective immunity. An effective vaccine stimulates both branches of the immune system to protect against disease.
Vaccines present foreign antigens from disease causing agents to a host so that the host can mount a protective immune response. Often vaccine antigens are killed or attenuated forms of the microbes which cause the disease. The presence of non-essential components and antigens in these killed or attenuated vaccines has encouraged considerable efforts to refine vaccine components including developing well-defined synthetic antigens using chemical and recombinant techniques. The refinement and simplification of microbial vaccines, however, has led to a concomitant loss in potency. Low-molecular weight synthetic antigens, though devoid of potentially harmful contaminants, are themselves not very immunogenic. These observations have led investigators to add adjuvants to vaccine compositions to potentiate the activity of the refined vaccine components.
Presently, the only adjuvant licensed for human use in the United States is alum, a group of aluminum salts (e.g., aluminum hydroxide, aluminum phosphate) in which vaccine antigens are formulated. Particulate carriers like alum serve to promote the uptake, processing and presentation of soluble antigens by macrophages. Alum, however, is not without side-effects and enhances humoral (antibody) immunity only.
An effective adjuvant potentiates both a humoral and cellular immune response in vaccinated animals. Further, an adjuvant must enhance a host's natural immune response and not aggravate the host system. A well-defined synthetic adjuvant free from extraneous matter which is stable and easy to manufacture would provide these qualities. Compounds that have been prepared and tested for adjuvanticity (Shimizu et al. 1985, Bulusu et al. 1992, Ikeda et al. 1993, Shimizu et al. 1994, Shimizu et al. 1995, Miyajima et al. 1996), however, often display toxic properties, are unstable and/or have unsubstantial immunostimulatory effects.
The discovery and development of effective adjuvants is essential for improving the efficacy and safety of existing vaccines. Adjuvants impart synthetic peptides and carbohydrate antigens with sufficient immunogenicity to insure the success of the synthetic vaccine approach. There remains a need for new compounds having potent immunomodulating effects.